The prior art as disclosed by Sato et al in U.S. Pat. No. 4,393,697 teaches the mounting of the mass air flow sensor directly on the main venturi member of the air intake system of an internal combustion engine. The main venturi member has a bypass passage connected at its downstream end to the low pressure region at the narrowed portion of the venturi. The sensing elements of the mass air flow sensor are disposed in this bypass passage and measures the air flow therethrough.
In an alternate embodiment taught by Sato et al, the body of the mass air flow sensor has a through air flow channel which is disposed directly into the main air flow of the air intake system. The sensing elements of the mass air flow sensor are located in the through air flow channel in the same manner as they are located in the bypass channel and the components of the electronic circuit are mounted on the external portion of the body of the mass air flow sensor.
In a current automotive application shown in FIG. 1, the mass air flow sensor 1 is mounted directly on a sensor housing 2 which forms part of the air intake or induction system of an internal combustion engine. The sensor housing 2 has a main air flow passageway 3 extending the length thereof and a smaller diameter air flow passageway 4 offset to one side of the main air flow passageway 3. The sensing head 5 of the mass air flow sensor 1 extends into the smaller diameter air flow channel 4 and detects the air flow therethrough. The sensor housing 2 also has a diametrically disposed internal wing 6 which produces a pressure differential across smaller diameter air flow channel 4 enhancing the air flow therethrough.
This mounting arrangement of the mass air flow sensor has several drawbacks. The sensor housing 2 with the integral smaller air flow channel 4 and the wing 6 is a complex part and relatively expensive to manufacture. Also, the manufacturing tolerances of the sensor housing 2 results in variations in the flow pattern in the smaller air flow channel 4. These variations in the flow pattern in the smaller air flow channel 4 adversely affect the calibration accuracy of the mass air flow sensor 1 once installed on a particular sensor housing 2.